One generally known anthropomorphic robot which has arms on both sides of the torso like the human being employs an arm joint structure as shown in FIG. 7 of the accompanying drawings.
The anthropomorphic robot has arms A each having a shoulder joint assembly a, an elbow joint assembly b, and a wrist joint assembly c. The shoulder joint assembly a comprises a first joint e coupled to a torso B for rotation about a first axis d which extends horizontally from an upper side of the torso B, a second joint g coupled to the first joint e for rotation about a second axis f (perpendicular to the sheet of FIG. 7) normal to the first axis d, and a third joint i coupled to the second joint g for rotation about a third axis h normal to the second axis f. The first through third joints e, g, i are positionally related to each other such that the first through third axes d, f, h intersect at one point in arbitrary operating positions (angular positions) of the first through third joints e, g, i.
The elbow joint assembly b comprises a fourth joint b coupled to the third joint i of the shoulder joint a by an upper arm k for rotation about a fourth axis j (perpendicular to the sheet of FIG. 7). The wrist joint assembly c comprises fifth through seventh joints p, q, r that are successively coupled to the elbow joint assembly b for rotation about respective fifth through seventh axes m, n, o (which are normal to each other in the illustrated state). A hand s is coupled to the distal end of the wrist joint assembly c.
The above structure of each of the arms A allows the shoulder joint assembly a, the elbow joint assembly b, and the wrist joint assembly c to operate in substantially the same fashion as human arms.
In the robot shown in FIG. 7, the elbow joint assembly b is positioned on the third axis h of the third joint i of the shoulder joint assembly a. However, the elbow joint assembly b may be coupled to the third joint at a position radially spaced from the third axis of the third joint.
With the arm A of the robot having the above structure, as shown in FIG. 8, the first axis d of the first joint e and the third axis h of the third joint i of the shoulder joint assembly a may be aligned with each other, and such an aligned state is referred to as a singularity state (or singularity posture). The singularity state refers to a state wherein the first axis d of the first joint e and the third axis h of the third joint i are aligned with each other irrespective of the angular displacement of the first joint e or the third joint i. This singularity state also occurs if the elbow joint assembly b is coupled to the third joint at a position radially spaced from the third axis of the third joint.
In the singularity state, the shoulder joint assembly a allows the first joint e or the third joint i to rotate only about the first axis d (=the third axis h) and also allows the second joint g to rotate only about the second axis f. Therefore, the elbow joint assembly b coupled to the third joint i of the shoulder joint assembly a has a reduced number of degrees of freedom for its posture, resulting in poor operability of the arm A.
For example, even for slightly changing the orientation of the fourth axis j of the elbow joint assembly b about the vertical axis in the singularity state shown in FIG. 8, it is necessary to operate the shoulder joint assembly a by rotating the first joint e and the third joint i of the shoulder joint assembly a by 90 degrees in opposite directions respectively about the first axis d and the third axis h, and thereafter rotating the second joint g slightly about the second axis f.
In the singularity state of the shoulder joint assembly a, therefore, the shoulder joint assembly a needs to be operated largely in order to slightly change the position or posture of the elbow joint assembly b, with the result that the flexibility and quickness to change the elbow joint assembly b to a desired position or posture.
The above shortcoming does not exist in a state other than the singularity state of the shoulder joint assembly a (non-singularity state). For example, in the state shown in FIG. 7, the orientation of the fourth axis j of the elbow joint assembly b can slightly be changed to a desired orientation by slightly rotating the first joint e or the third joint i or both.
The inventors have found that the arms A of the anthropomorphic robot of the type described above operate to perform various processes while the elbow joint assembly b is positioned at the same height as or lower than the center of the shoulder joint assembly a, i.e., the point of intersection of the axes d, f, h of the first through third joints e, g, i. This tendency is particularly strong for master-slave anthropomorphic robots whose arms are operated in the same manner as human arms, because of the working ability of the human arms.
With the conventional anthropomorphic robot, however, the layout and posture of the first through third joints of the shoulder joint assembly a have been designed taking into consideration the position of the shoulder joint assembly b of the arm A in the actual operation. Therefore, the singularity state of the shoulder joint assembly a occurs in positions where the elbow joint assembly b can operate when the anthropomorphic robot works. With the anthropomorphic robot shown in FIG. 7, for example, the singularity state occurs when the elbow joint assembly b extends laterally of the torso B at the same height as the center of the shoulder joint assembly a. Such a position of the elbow joint assembly b normally occurs while the anthropomorphic robot is working. Consequently, while the arm A of the conventional anthropomorphic robot is normally working, the singularity state of the shoulder joint assembly a is liable to take place, resulting in impaired operability of the arm A during normal operation of the robot.
In view of the above background, it is an object of the present invention to provide an arm structure for anthropomorphic robots which minimizes the occurrence of a singularity state of a shoulder joint assembly while the arm of the anthropomorphic robot is normally working, for thereby allowing the arm to operate smoothly.